The invention relates to a method designed to enable at least a portion of a string of tools permanently installed in an oil well such as a production well to be saved in the event of a fault localized on one of the tools or on an electric cable connecting a surface installation to each of the tools in series.
The invention also relates to a transmission assembly including an electric cable connecting a surface installation in series to each of the tools of a string of tools installed permanently in an oil well, and organized so as to enable such a method to be implemented.
In an oil well, a string of tools is generally installed permanently for the purpose of measuring and controlling various characteristic magnitudes of the well. Such a string of tools comprises a certain number of tools disposed in the well one above another at determined depths. Each tool is independent of the others and is provided in particular with means for measuring one or more magnitudes, or with means for controlling the flow of petroleum fluid along the well, or indeed with measurement means and with associated control means.
As a general rule, an electric cable connects a surface installation in series to each of the tools in a string of tools constituted in this way. The electric cable serves both to feed electricity to each of the tools and to transmit electrical signals from the surface installation to each of the tools, and vice versa.
In an existing arrangement, the link comprises two twisted conductors housed in a stainless steel tube. In the surface installation, and also in each of the tools, signals are transmitted and received via a transformer. Electrical power is then supplied in common mode.
In that arrangement, which is characterized by a serial electrical link between the surface installation and the various tools, an interruption of an electrical conductor or the appearance of a short circuit will have the consequence of making the entire string of tools inoperative.
U.S. Pat. No. 4,398,271 describes a system for transmitting data in a seismic streamer comprising a number of links, each of which includes switch means. In use, the switch means are set to one of a number of possible positions to isolate discontinuities and form a continuous data path.
A specific object of the invention is to provide a method enabling at least a portion of a string of tools installed in an oil well to be saved in spite of a short circuit or an interruption occurring in the cable or in one of the tools.
According to the invention, this result is obtained by means of a method of saving a string of tools installed in an oil well in the event of a localized fault, a link connecting a surface installation in series with each of the tools of said string, the method being characterized in that it consists in subdividing the link into segments that are successively interconnected by switch means, in locating a fault, if any, by closing each of the switch means one after another starting from the surface installation until said fault appears, and in closing only those switch means that precede the last-closed switch means.
Because of the presence of switching means on the link, it becomes possible to save that portion of the string of tools which is situated above the location where the fault has occurred, by leaving open the lowest switching means connecting the surface installation to the faulty tool or segment.
In a preferred implementation of the invention, a link is used that forms a loop between the surface installation and the tools. Said fault is located by closing the switch means in both directions along said loop from the surface installation, and in each direction only the switch means preceding the last-closed switch means are closed.
Preferably, switch means are associated with each of the tools.
Advantageously, each tool is caused to communicate separately with each of the two segments that are interconnected by the switch means associated with said tool.
By means of these characteristics in combination, it is possible either to keep all of the tools in operation when the fault has occurred on a segment interconnecting two tools, or else to put only one of the tools out of circuit when the fault has occurred within said tool.
In a preferred implementation of the invention, the link comprises an electric cable. In which case, advantageously, the tools are powered in common mode while signals are simultaneously being interchanged between the surface installation and each of the tools over a two-conductor cable.
The invention also provides a link connecting a surface installation in series with each of the tools of a string of tools installed in an oil well, which assembly is characterized in that the link is made up of successive segments that are interconnected by switch means suitable for being opened to isolate at least one faulty tool or cable segment.